Mass spectrometer having means compensating electron transit time across the cathode of the electron multiplier

ABSTRACT

To improve the resolution of a mass spectrometer the emissive surface of the cathode for the electron multiplier is precisely angled with respect to the plane perpendicular to the axis of the ion focusing tube so that all ions of the same mass which pass through that plane at the same time impinge on those portions of the cathode surface closest to and farthest from the dynode cause electrons secondarily emitted therefrom to impinge upon the dynode at substantially the same time. This shortens the duration of the mass spectrum signals making it easier to distinguish one mass-spectrum line from another in the 200-500 amu range.

United States Patent Damoth et al.

[451 May2, 1917 [54] MASS SPECTROMETER HAVING MEANS COMPENSATING ELECTRON TRANSIT TIME ACROSS THE CATHODE OF THE ELECTRON MULTIPLIER [72] Inventors: Donald C. Damoth, Rochester, N..Y.; William H. Shriner, Blanchester, Ohio [73] Assignee: The Bendix Corporation [22] Filed: Sept. 15, 1969 [21] Appl.No.: 858,058

[56] References Cited UNITED STATES PATENTS 2,762,928 9/1956 Wiley ..250/4l.9

Primary ExaminerWilliam F. Lindquist Attorney-Plante, Arens, Hartz, Hix and Smith [57] ABSTRACT To improve the resolution of a mass spectrometer the emissive surface of the cathode for the electron multiplier is precisely angled with respect to the plane perpendicular to the axis of the ion focusing tube so that all ions of the same mass which pass through that plane at the same time impinge on those portions of the cathode surface closest to and farthest from the dynode cause electrons secondarily emitted therefrom to impinge upon the dynode at substantially the same time. This shortens the duration of the mass spectrum signals making it easier to distinguish one mass-spectrum line from another in the 200-500 amu range.

2 Claims, 2 Drawing Figures PATENTEDMAY 2 I972 m WEDGE DONALD C. DA MOTH WILLIAM H. SHRINER INVENTORS MASS SPECTROMETER HAVING MEANS COMPENSATING ELECTRON TRANSIT TIME ACROSS THE CATHODE OF THE ELECTRON MULTIPLIER BACKGROUND OF INVENTION This invention relates to mass spectrometers (e.g. U. S. Pat. No. 2,765,408) and more specifically to the physical relationship between the electron multiplier cathode, which collects ions accelerated through the drift tube, and the drift tube and/or grid or screen through which the ions pass before impinging upon the multiplier cathode.

The mass spectrometer is an instrument that permits rapid analysis of chemical compounds by measurement of one of the most basic parameters of matter, atomic mass units. In operation, a small amount of the gas to be studied is admitted into the vacuum tube of the spectrometer through a sample inlet. The gas is then ionized by electrons emitted from a filament and speeded up by an accelerating grid. An electric field draws the ions out of the ionizing region and into a region where the ions are separated according to their ratios of mass to charge (m/e). The ions them impinge upon the cathode of an electron multiplier to achieve a gain of 10 or greater. The resulting output signal is then transmitted to a read out device which indicates the mass spectrum. The aforementioned separation of ions may be accomplished by various means, such as deflection with an electric or magnetic field. In a timeof-flight mass spectrometer, separation of ions is accomplished by subjecting a packet of various ions to a negative accelerating potential over a predetermined distance which causes the light ions to be accelerated faster than the heavier ions so the lighter ions reach the cathode of the electron multiplier earlier than the heavy ions. A drift tube for containing the ion packet supplies the distance required to separate ions according to their mass to charge ratio. Since the time interval required for a particle to travel the length of the drift tube depends on the ratio of its mass to its atomic charge every element or compound has a characteristic mass-spectrum line at the spectrometer output.

The output signal from a mass spectrometer is generally introduced into a readout device such as an oscilloscope or strip chart recorder from which the mass spectrum lines can be read to determine the character of the sample introduced into the spectrometer. The degree to which the mass spectrum lines are distinguishable on the chart is called resolution. Resolution between ions in the -200 amu mass range for spectrometers has not been a problem. However, ions having a mass in the 200-500 amu range are not easily distinguished from each other on the chart because there is not always sufficient spacing between the different mass-spectrum lines. The spacing being a function of the time duration of and between output signals from the electron multiplier (output mass spectrometer). Often appearing on the chart is the 200-500 amu range are marks or blobs having a width which is twice the width or larger than the normal width of a mass-spectrum line. Obviously interpreting the meaning of these blobs is often impossible. The blobs are generally caused by overlapping output signals (pulses) from different ions the duration of which is a function of the time it takes for all the ions of a given mass to impinge upon the cathode of the electron multiplier and travel to the output of the multiplier. Prior art time of flight mass spectrometers advanced to the point where it was possible to have substantially all the ions of a given mass strike the cathode at approximately the same time. This was accomplished by maximizing the length of the drift tube and arranging a gate or screen grid perpendicular to the longitudinal axis of the drift tube to arrange the ions in a plane one molecule thick. However, since resolution was considered as dependent on time rather then geometric factors no further improvement with respect to shortening the time interval of the mass-spectrum lines at the output was believed possible. However, the demands of the biological and organic sciences require better resolution in the 200-500 amu range making output signals having a signal duration 5 to nanoseconds shorter than the normal signal duration important. Such an improved signal would make it possible to distinguish between those massspectrum lines of ions in the 200-500 amu range which were not previously distinguishable.

SUMMARY OF THE INVENTION To improve resolution between mass spectrum lines in the 200-500 amu range of a time of flight mass spectrometer, the transit time across the electron multiplier cathode of all electrons emitted by ions of the same mass number is compensated for by disposing the cathode at an angle to a plane which is perpendicular to thelongitudinal axis of the drift tube. The invention is characterized by the fact that substantially all the electrons propagated from the cathode by impingement of ions having the same mass strike the dynode at approximately the same time. The arrangement is especially useful in improving the resolution of mass spectrometers of the type described in US. Pat. Nos. 2,685,035 and 2,765,408 because it decreases the time interval for all the electrons, emitted from the cathode by impinging ions of the same mass, to strike the dynode of the electron multiplier.

Accordingly, it is an object of this invention to improve the resolution of mass-spectrum lines in the 200-500 amu range.

It is another object of this invention to decrease the time interval it takes electrons emitted from the cathode by impinging ions of the same mass to strike the dynode.

It is another object of this invention to improve the performance of time of flight mass spectrometers.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of a time of flight mass spectrometer.

FIG. 2 shows a preferred arrangement of the cathodeand dynode of an electron multiplier with respect to a grid through which ions pass before impinging the cathode.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 shows a mass spectrometer of the type described in U.S. Pat; Nos. 2,765,408 and 2,685,035. Molecular species entering the ionizing region 1 from the sample inlet 3 are ionized by electrons from filament 5. The ions 10 are then accelerated into the drift tube 7 by accelerating grids 9. Because of the length of the drift tube 7 and the different velocities of the ions, the ions are separated according to their mass to charge ratio (m/e) striking the cathode 20 at different times. The different times of flight (T) for ions of different masses (m)' to travel from the ionizing region to the cathode may be calculated mathematically from the equation T=k(m/e)"", where k is a constant depending on physical dimensions. For example, the time of flight of a singly charged nitrogen ion m=28 atomic mass units in the Bendix time of flight mass spectrometer is 5 microseconds and under usual conditions the time width of the nitrogen pulse at the spectrometers output is about 0.015 microsecond. (about 0.060 microsecond for atomic species of 400 amu) A magnetic electron multiplier 30 (e.g. U. S. Pat. Nos. 2,983,845 and 2,841,729) is used to detect and amplify the ion bunches. The ions pass through aperture 2 of grid 4 and strike the cathode 20 to produce secondary electrons 40. The cathode 20 is disposed in a plane that makes an acute angle with a plane that is perpendicular to the longitudinal axis of the drift tube 7. This arrangement decreases the time interval for all the electrons, propagated from the cathode by ions of the same mass, to strike the dynode. The electrons follow a cycloidal path under the influence of the mutually perpendicular electric and magnetic fields in the multiplier to strike the dynode strip 31 multiplying in number to achieve a gain of approximately 10. The resulting output signal is then synchronized on an oscilloscope, or gated (gates 35) to an analog (not shown) for strip chart recording.

FIG. 2 shows a group of ions l0, ll, 12 of the same mass traveling in a direction towards the cathode 20 of a magnetic electron multiplier 30 (e.g. U. S. Pat. Nos. 2,84l,729 and 2,932,768). The length of the cathode surface exposed to ionizing ions is designated d The broken line 40, 41, 42 indicates the path of secondary electrons traveling towards the dynode 31. Although the dynode shown is located in a plane that faces the same general direction, the dynode may be located such that a portion of the dynode is located in a plane that is perpendicular to the cathode plane. The distance between the grid 4 and cathode 20 at different points along the cathode surface is indicated by D and D where D is the distance between a first point on the cathode 20 and the grid 4, and D is the distance between the cathode and grid at a second point on the cathode surface which is closer to the dynode 31 than the first point. To achieve the objects of the invention it is preferred that the ratio D /D be less than 1. The cathode 20 is disposed at an angle A which is defined by the intersection of the plane 50 in which the cathode 20 is located and the plane 60 which is perpendicular to the longitudinal axis 8 of the drift tube 7. Angle A is an acute angle, which for a timev of flight mass spectrometer is preferably an angle having a tangent less that 0.020 and greater than 0.005 and is calculated to be the angle at which cathode 20 should be oriented so that secondary electrons, propagated by an ion furthest from the dynode 31, strike the dynode at the same time that electrons emitted by an ion closest to the dynode strike the dynode. For example, angle A is such that the electronsemitted by ion 10 travel along path 40 and strike the cathode surface at the same time ion 11 does. Then the electrons emitted from the cathode surface follow path 41 and strike the cathode surface at the same time ion 12 strikes the cathode surface. [on 12 having traveled the additional distance 01, than ion 10 to reach the cathode 20. This action permits all the electrons emitted from the cathode surface by ions of the same mass to leave the cathode surface and impinge upon the dynode 31 at substantially the same time. It should be noted that the cathode 20 must be precisely located with respect to the grid 4 and the dynode 31 otherwise the resolution of the spectrometer will be adversely affected.

In operation, a gas sample to be analyzed is introduced into the mass spectrometer where it is ionized and accelerated through the drift tube 7. Because of the different velocities and the length of the drift tube 7 the ions are separated into groups according to their mass to charge ratio (m/e). At the end of the drift tube is a grid 4 which defines the end of the drift region and the beginning of the field between the grid 4 and the cathode 20. All the ions 10, 11, 12 leaving the grid plane at the same time now proceed towards the cathode 20. Because the cathode 20 is precisely disposed at an angle A the ions (e.g. 12) closest to the dynode 21 travel an additional distance d, to reach the cathode 20 than ions (e.g. 10) furthest away from dynode 31. Since the ion 12 travels an additional distance d, it requires additional time t, to reach the cathode 20. This additional time t, is equal to the same time t it takes electrons secondarily emitted from the cathode as a result of ion 10 striking the cathode to reach the point where ion 12 strikes the cathode 20. As a result of this precisely predetermined time delay for some ions to strike the cathode 20 all the electrons propagated by ions of the same mass leave the cathode 20 for the dynode 31 at substantially the same time. This decreases the time interval required for all the electrons propagated by ions of the same mass to impinge upon the dynode 31. It also improves the resolution between mass spectrum lines at the spectrometer output because the time interval between mass-spectrum lines is increased. For a more detailed explanation of how the precise angle A is determined for cathode 20 in a particular apparatus the following example is offered:

EXAMPLE I To determine mathematically the additional distance d. that the ion closest to the dynode 31 should travel to the cathode 20 so that electrons propagated by ions 10, l l, 12 of the same mass strike the dynode 21 at the same time. Consider the following calculations Let t, The time required for an ion to travel the extra distance d Let t The time required for an electron to travel the distance d,

Since (velocity) (time) =Distance i r/ r Equation( I and 1 d lv Equation( 2) Required:

Make (t, t which expressed in terms of the distance and velocity of Equations 1 and 2 is:

t/ i e/ e i e i/ e The tangent of angle A is d ld or Tan A vf/v Equation( 3) Solving for v The velocity of an electronin a uniform electric and magnetic field is given by the following equation (See The Review of Scientific Instruments," Vol. 22, No. 3, Mar. 1951, p 166) v =c/3OO H E X H where c Velocity oflight 3 X10 cm/sec H Intensity of magnetic field (gauss) E= Electric field intensity (volts/cm) for E perpendicular to H E X H EH by substitution the equation reduces to:

V E/H(10 cm)/sec Equation(4) Expressing E in terms of cathode and grid voltage and the distance therebetween where V Voltage at cathode (volts) V,,= Voltage at grid (volts) d Distance between cathode and grid (cm) by substitution Equation 4 becomes v V V,, /dH l0 cm)/sec Equation( 5) Solving for v, The velocity of an ion can be derived from the following equation (See The Review of Scientific Instruments, Vol. 26, No. 12, Dec. 1955, p 1,152) Assuming the ion is traveling at a constant velocity T l.02D(2m)" /2 U where m mass (amu) D= distance traveled (cm) U terminal energy (ev) T= time (psec) which reduces to TB 0.72.0 "E /U Equation(6) which in terms of velocity V= D/Tis D U 1.39U cm. vi .72Dm .72m in ,u see.

which in terms of seconds is v 1.39 U /m" l0 cm/sec and by substituting U Voltage at cathode (volts) v l.39c" /m l0 cm/(sec) E uation(7) From Equation 3 Tan A v lv Equation( 7 )/Equation( 5) 1.39V cm.

m sec.

/V,,Vg/ 10 cm.

i W sec.

Tan A:

Based upon the parameters of a given system, e.g. a Bendix time of flight mass spectrometer, a table of .values may be ob- From the above table it can be seen that to improve the resolution for a particular mass-spectrum line the cathode may be angled as indicated with respect to the grid. For example, the resolution of the mass-spectrum line for a molecule having a mass of 400 amu is improved by placing the cathode at an angle A with the grid such that the tangent of the angle is 0.0091.

While a preferred embodiment of the invention has been disclosed it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some, cases, certain features of the invention may be used to advantage Without corresponding use of other features. For example, it can be seen from equation 8 that the angle A that the cathode must make with the dynode to have t, t is dependant upon the physical and electromagnetic relationships of a particular system which for a particular system may require angle A to be quite large. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention, what is claimed is:

1. In combination with a time of flight mass spectrometer of the type having means for producing ions; means for accelerating the ions produced; a drift tube for receiving the accelerated ions; a grid disposed at the exit end of said drift tube and perpendicular to the longitudinal axis of said drift tube; and an electron multiplier for collecting the ions after their travel through said drift tube and exit from said grid, said electron multiplier including a cathode and a dynode, and wherein the ions produced are accelerated through said drift tube for separation into groups according to their mass to charge ratio (m/e) and wherein the ions impinging said cathode disposed at one end of said drift tube cause electrons to be propagated from said cathode and onto said dynode of said electron multiplier, the improvement wherein said cathode is disposed in a plane that makes an acute angle of about 12 with a plane that is perpendicular to the longitudinal axis of said drift tube such that the ratio 1 2 is less than 1, where D is the distance between said grid and a first point on said cathode impinged by an ion, and D is the distance between the grid and a second point on said cathode impinged by another ion, said second point on said cathode being closer to said dynode than said first point so that all the electrons propagated from the entire exposed area of said cathode, by impingement of a group of ions having the same mass, strike the dynode at the same time.

The combination of claim 1 wherein said dynode is disposed in a plane that makes an acute angle of about 12 with the plane of said cathode. 

1. In combination with a time of flight mass spectrometer of the type having means for producing ions; means for accelerating the ions produced; a drift tube for receiving the accelerated ions; a grid disposed at the exit end of said drift tube and perpendicular to the longitudinal axis of said drift tube; and an electron multiplier for collecting the ions after their travel through said drift tube and exit from said grid, said electron multiplier including a cathode and a dynode, and wherein the ions produced are accelerated through said drift tube for separation into groups according to their mass to charge ratio (m/e) and wherein the ions impinging said cathode disposed at one end of said drift tube cause electrons to be propagated from said cathode and onto said dynode of said electron multiplier, the improvement wherein said cathode is disposed in a plane that makes an acute angle of about 1.2* with a plane that is perpendicular to the longitudinal axis of said drift tube such that the ratio D1 / D2 is less than 1, where D1 is the distance between said grid and a first point on said cathode impinged by an ion, and D2 is the distance between the grid and a second point on said cathode impinged by another ion, said second point on said cathode being closer to said dynode than said first point so that all the electrons propagated from the entire exposed area of said cathode, by impingement of a group of ions having the same mass, strike the dynode at the same time.
 2. The combination of claim 1 wherein said dynode is disposed in a plane that makes an acute angle of about 1.2* with the plane of said cathode. 